The invention relates to the use of titanium based composites for use in component manufacture, or for use as coatings, with improved resistance to high-temperature oxidation and spallation. The invention also relates to titanium-based composites and to methods of production.
Titanium based alloys and intermetallic compounds are, in general, known. For example, titanium aluminide (Ti3Al) based intermetallic compounds are attractive structural materials for applications in the aerospace and automobile industries because of their low density, high melting point, and high specific strength and excellent mechanical properties. However, the industrial application of such materials has been limited.
Ti3Al based intermetallic compounds show a relatively low ductility and poor high-temperature oxidation resistance. Temperatures over approximately 650xc2x0 C. cause cracking to the oxide layer of these Ti3Al based compounds. This leads to spallation of the outer oxide layer of the compounds and rapid oxidation of the underlying material. Continued exposure of the Ti3Al based compounds to such environments eventually leads to the degradation and destruction of the materials. For these reasons, Ti based alloys and intermetallic compounds have been restricted in application to temperatures below 650xc2x0 C., as at about this temperature the materials become oxidised quickly.
Al3Ti coating, obtained using conventional pack cementation, can improve the oxidation resistance of Ti3Al, because the outer oxide layer or scale formed is composed mainly of xcex1-Al2O3. However, such coatings are not ideal as they form small but permeable cracks, which penetrate the coating layers and compromise the oxidation resistance. As a result the materials have limited applications.
To improve the mechanical and oxidation properties, Ti3Al has been alloyed with Nb, Cr, Mo, Si and/or W and this has shown some benefits. The main problem with alloying methods is that one special element cannot improve all required properties to a desirable level. Multi-element alloying is therefore often used, and microstructural control with thermal or thermo-mechanical treatment is required in most cases. Such methods are complicated and expensive.
It will be appreciated by those skilled in the art that if titanium based materials are to have a wider range of commercial applications at high temperatures, they must be substantially resistant to oxidation and spallation at high temperatures; easy to prepare and fabricate; and be cost efficient.
With the above background in mind, it is an object of this invention to provide titanium based composite materials which address or at least ameliorate disadvantages of known titanium based alloys and intermetallic compounds, or at least which will provide the public with a useful alternative.
Further objects of this invention will become apparent from the following description which is given by examples only.
According to one aspect of this invention there is provided a titanium based composite which includes a Ti(Al,O) base matrix, discrete ceramic particles, and an oxide layer on the surface of the composite, wherein the discrete ceramic particles are integrally associated with the Ti(Al,O) base matrix and the oxide layer, and wherein, at a temperature of above about 600xc2x0 C., the composite is substantially resistant to oxidation and/or spallation.
Preferably the discrete ceramic particles range in size from 0.1 xcexcm to 30 xcexcm.
Preferably the discrete ceramic particles are selected from Al2O3, SiC, TiC, TiN, TiB2, Y2O3 and/or Si3N4.
Preferably the discrete ceramic particles may constitute a volume fraction of about 10% to 60% of the titanium based composite.
According to a further aspect of this invention there is provided a coating material including titanium based composite adapted for use on substrate components used at high temperature and/or in oxidative environments, wherein the composite includes a Ti(Al,O) base matrix, discrete ceramic particles and an oxide layer, wherein the discrete ceramic particles are integrally associated with the Ti(Al,O) base matrix and the oxide layer so that at a temperature of above about 600xc2x0 C. the composite is substantially resistant to oxidation and/or spallation.
Preferably the discrete ceramic particles range in size from 0.1 xcexcm to 30 xcexcm.
In one preferred form the discrete ceramic particles are selected from Al2O3, TiC, SiC, TiN or TiB2.
Preferably the ceramic particles constitute a volume fraction of about 10% to 60% of the titanium based composite.
Preferably the composite is resistant to oxidation and/or spaliation at temperatures between 600xc2x0 C. and 900xc2x0 C. and more preferably above 700xc2x0 C.
According to a further aspect of this invention there is provided a method of producing a coating for application to a component used at temperatures above 600xc2x0 C. and/or in oxidative environments, wherein the method includes the steps of:
preparing a Ti(Al,O) based composite powder, with each of the powder particles including discrete Al2O3 particles, according to the mechanical milling and thermal treatment method disclosed in PCT/NZ98/00124;
applying the composite powder produced to a substrate component to produce a composite coating; and
exposing the coated component to a high temperature, oxidative environment above about 600xc2x0 C. to form a surface oxide layer on the composite coating.
Preferably the composite powder is applied to the substrate using a thermal or plasma spray process.
Preferably the coated component is heated to between about 700xc2x0 C. and about 900xc2x0 C. for between about 1 and 200 hours in an oxygen containing environment to form the surface oxide layer.
Preferably the coated component is heated in an oven before use or is heated in situ during use.
According to a further aspect of the invention, there is provided a process for producing a titanium based composite material in a pre-selected form including the steps of:
preparing a Ti(Al,O) based composite powder with each of the powder particles, including discrete Al2O3 particles, according to the mechanical milling and thermal treatment method disclosed in PCT/NZ98/00124;
pressing the powder formed into a pre-selected mould to produce a powder compact and sintering the powder compact at a temperature of above about 700xc2x0 C. under an inert environment;
exposing the sintered composite material or component to a high temperature, oxidative environment above about 700xc2x0 C. to form a surface oxide layer;
wherein the product produced is substantially resistant to oxidation and/or spallation at temperatures above 600xc2x0 C.
Preferably the sintering temperature is between 700xc2x0 C. and 1650xc2x0 C.
Preferably the inert environment is a vacuum or argon environment.
According to a further aspect of this invention there is provided a method of producing a coating for application to a component used at temperatures above 600xc2x0 C. and/or in oxidative environments, wherein the method includes the steps of:
preparing a Ti(Al,O) based composite powder, with each of the powder particles including discrete TiC, SiC, TiN, TiB2, Y2O3 and/or Si3N4 particles, according to the mechanical milling method disclosed in PCT/NZ98/00124;
applying the composite powder produced to a substrate component to produce a composite coating; and
exposing the coated component to a high temperature, oxidative environment above about 600xc2x0 C. to form a surface oxide layer on the composite coating.
Preferably the composite powder is applied to the substrate using a thermal or plasma spray process.
Preferably the coated component is heated to between about 700xc2x0 C. and about 900xc2x0 C. for between about 1 and 200 hours in an oxygen containing environment to form the surface oxide layer.
Preferably the coated component is heated in an oven before use or is heated in situ during use.
Preferably the component is to be used at temperatures between 600xc2x0 C. and 900xc2x0 C.
According to a further aspect of the invention, there is provided a process for producing a titanium based composite material in a pre-selected form including the steps of:
preparing a Ti(Al,O) based composite powder, with each of the powder particles including discrete TiC, SiC, TiN, TiB2, Y2O3 and/or Si3N4 particles, according to the mechanical milling method disclosed in PCT/NZ98/00124;
pressing the powder formed into a pre-selected mould to produce a powder compact and sintering the powder compact at a temperature of above about 700xc2x0 C. under an inert environment;
exposing the sintered composite material or component to a high temperature, oxidative environment above about 700xc2x0 C. to form a surface oxide layer;
wherein the product produced is substantially resistant to oxidation and/or spallation at temperatures above 600xc2x0 C., preferably between 600xc2x0 C. and 900xc2x0 C.
Preferably the sintering temperature is between 700xc2x0 C. and 1650xc2x0 C.
Preferably the inert environment is a vacuum or argon environment.
Other aspects of the invention will become clear from the disclosure below which is given by way of example only with reference to the figures.